This PR seeks to introduce affiliate codes in Jetpack creating a class that allows to get an affiliate code from a database option jetpack_affiliate_code, and filtering it. There are two ways to set this code in the website:

1. Partners that can setup an affiliate code through WP CLI when deploying a site, can issue:
   wp option add jetpack_affiliate_code abc123
2. other partners like agencies that don't have this control from the beginning can add a filter:
   add_filter( 'jetpack_affiliate_code', function() { return 'abc123'; } );

Additionally, this PR will also:

• add the affiliate code, if found, to the connect url, as the query parameter aff
• send it to Jetpack UI in the Initial_State object as the property aff
  it will be added to upgrade links in a follow up PR, since we need to refactor how upgrade links are written (right now they're inline) to be able to add the affiliate code or another param in the future in a consistent way using a function that given some params, returns the upgrade link.

Testing instructions

a. manually

1. do wp option add jetpack_affiliate_code abc123 (or yarn docker:wp option add jetpack_affiliate_code abc123)
2. verify that the URL of the Set up Jetpack button has the aff parameter with the right value
3. verify that the Initial_Setup.aff property has the right value

b. running unit tests. There's a new test suite named affiliate

1. run phpunit --testsuite=affiliate or yarn docker:phpunit --testsuite=affiliate

Proposed changelog entry

Not necessary to mention this change since it will be known for those that have to know about it.